Polished, elongated rectangular luminescent elements being irradiated by blue light from LEDs are well known within the field of light emitting devices. Within such a luminescent element, the blue light is converted into green or red light. Photons within a certain directional cone will experience total internal reflection (TIR) by the sides of the luminescent element, and will propagate to the exit window (or to a mirror on the surface opposite to the exit window) and can be used for a desired imaging or lighting purpose.
The conversion of the blue light into light of longer wavelengths is, due to Stokes shift, associated with heat generation. The heat has to be transported out of the luminescent element while maintaining the TIR for the light. This can be performed by maintaining a small, but nonzero distance between the TIR surface and a surface of a cooling plate.
WO 2014/202726 A1 describes a light emitting device comprising a transparent heat sink element and a luminescent light guide. In connection with one embodiment it is mentioned that a gap may be provided between the transparent heat sink element and the luminescent light guide. The gap may have a size of e.g. less than 200 μm, less than 100 μm or less than 50 μm. The gap may be an air gap maintained by means of spacer elements, or it may be partially or completely filled with an optically transparent and thermally non-transparent material such as an optically transparent adhesive with a relatively low thermal conductivity.
However, as the distance between the luminescent element and the cooling element contributes something like 50% of the total thermal resistance, the magnitude of the distance is critical. In particular, it is important that the distance between the luminescent element and the cooling element is large enough that the evanescent wave created in connection with the TIR does not cause light losses towards the cooling plate, yet not too large as it will then contribute too much to the thermal resistance. Maintaining this correct distance in an industrial environment has turned out to be a challenging task.